The present invention in accordance with certain embodiments, relates to a nondestructive testing technique for determining thickness, as well as conductivity of an insulative coating. More particularly, the present invention provides a high-speed infrared transient thermography method and apparatus for measuring thickness and conductivity for an insulative coating.
Over the years, various nondestructive ultrasonic measurement techniques have been utilized to determine the cross-sectional thickness of cast metal or other solid objects. Unfortunately, conducting ultrasonic measurements to examine the cross-sectional thickness generally necessitates a cumbersome and time-consuming mechanical scanning of the entire surface with a transducer. In addition, to facilitate intimate sonic contact between the transducer and the object surface to provide proper propagation of ultrasonic waves into the object, a stream of liquid couplant must be applied to the surface, or, alternatively, total immersion of the object in the couplant must be accommodated. Such accommodations, however, are not practical or even feasible for numerous structural and material reasons. Moreover, ultrasonic systems capable of scanning and analyzing geometrically complex parts are typically very expensive and complicated. In addition, a mechanical scanning of the transducer over the surface of a large object can be a time consuming process, increasing costs and production times.
In contrast, infrared (IR) transient thermography is a somewhat more versatile nondestructive testing technique that relies upon temporal measurements of heat transference through an object to provide information concerning the structure and integrity of the object. Heat flow through an object is substantially unaffected by the micro-structure and the single-crystal orientations of the material of the object, therefore, an infrared transient thermography analysis is essentially free of the limitations this creates for ultrasonic measurements. Additionally, transient thermographic analysis approach is not significantly hampered by the size, contour or shape of the object being tested and, moreover, can be accomplished ten to one-hundred times faster than most conventional ultrasonic methods, particularly when testing objects with large surface areas.
Conventionally, an infrared (IR) video camera has been used to record and store successive thermal images (frames) of an object surface after heating it. Each video image is composed of a fixed number of pixels, each pixel representing a small picture element in an image array or frame. Each pixel corresponds to a rectangular area, called a resolution element, on the surface of the object being imaged. Because, the temperature at each resolution element is directly related to the intensity of the corresponding pixel, temperature changes at each resolution element on the object surface can be analyzed in terms of changes in pixel contrast.
One known contemporary application of transient thermography is the ability to determine the size and relative location (depth) of flaws within solid non-metal composites; another application of transient thermography is for determining the thickness of metal objects. Some attempts have been recently made to measure the thickness of insulative coating as well. These include modeling techniques where the insulative coating thickness may be obtained by fitting the coating data to a model and comparing it with known thickness standards. Unfortunately, these techniques include point-by-point measurement of the coating thickness, and therefore take time and are complex computationally. Another aspect to thickness measurement for insulative coatings is that as the coating ages the thermal conductivity of the coating changes and affects the thickness of the coating. Therefore, there is also a need for determining the thermal conductivity as well for the insulative coating for accurate thickness measurement.
Therefore, there is a need for a technique that can measure quantitatively, the absolute thickness for insulative coating without using the thickness standards.